intensities zero in vibrational analysis. why?

Hi,

I have got a question about calculation of vibrations of molecules and estimate intensities.

I energy minimised an organic molecule (PTCDI) using DMol3, which creates the hessian of the molecule.

Then I go to Tools->Vibrational analysis and click to calculate. It gives many different vibrational modes, however all the intensities are zero, I wonder why.The animation and spectrum functions also give no animation or all points at y=0 respectively.